1. Field of the Invention
The present invention relates to a power unit for driving a driven member, and more particularly to a power unit having a prime mover and rotating machines as power sources.
2. Description of the Related Art
Conventionally, as a power unit of this kind, one disclosed e.g. in Japanese Laid-Open Patent Publication (Kokai) No. 2002-281607 is known. This power unit is for driving drive wheels of a vehicle, and is provided with an internal combustion engine, a first rotating machine, and a second rotating machine, which are power sources, a first planetary gear unit and a second planetary gear unit, and a first control unit and a second control unit, which are for controlling the first and second rotating machines. A carrier of the first planetary gear unit and a sun gear of the second planetary gear unit are connected to each other and are connected to the engine. A ring gear of the first planetary gear unit and a carrier of the second planetary gear unit are connected to each other and are connected to the drive wheels. Further, a sun gear of the first planetary gear unit and a ring gear of the second planetary gear unit are connected to the first and second rotating machines, respectively. Furthermore, the first and second control units are implemented e.g. by inverters.
In the power unit configured as above, the drive wheels are driven using power from the engine and the first and second rotating machines according to traveling conditions of the vehicle. Further, power generation is performed by the first rotating machine using part of power from the engine under the control of the first and second control units, to directly supply the generated electric power to the second rotating machine, and the electric power generated by the first rotating machine, and the rotational speeds of the first and second rotating machines are controlled to thereby transmit power from the engine to the drive wheels while changing the speed of the power from the engine.
As described above, to change the speed of the power from the engine, the conventional power unit is required to incorporate two pairs of the rotating machine and the control unit, which results in an increase in the size of the power unit and an increase in manufacturing costs thereof. Further, to transmit power from the engine to the drive wheels while changing the speed of the power from the engine, it is necessary to control the electric power generated by the first rotating machine and the rotational speeds of the first and second rotating machines in a fine-grained manner, which causes an increase in computation load on the first and second control units.